Technical Abstract:
The objective of this study was to determine the effect of age of the ovulatory follicle on fertility. Ovulation (d 0) was synchronized in post pubertal heifers in Montana (n = 153; MT) and Ohio (n = 152). All heifers received estradiol benzoate (EB; 1mg/500kg BW) on d 6 and were assigned to either receive PGF (25 mg, i.m.) 5 d (d 11 of the experiment; “young” follicle, YF, n = 154) or 9 d (d 15 of the experiment; “mature” follicle, MF, n = 151) after EB. Estrus was detected for 5 d after PGF with AI approximately 12 h after estrus. Ovarian ultrasonography (MT only) was performed on d 6, 11, 15 (MF only) and at AI. Heifers that failed to initiate a new follicular wave after EB (MT only) were excluded from further analyses (n = 11). Heifers from the MF treatment in MT that initiated a second follicular wave after EB, but before PGF on d 15 (n = 14, MF2) remained in the analyses. Estrous response and conception rate were analyzed using a model that included location, treatment, and their interaction, with the GLIMMIX procedure of SAS. Interval from PGF to estrus was analyzed with the MIXED procedure of SAS. In a second analysis, heifers with two follicular waves after EB (MF2) were compared to the MF heifers with one wave, and the YF treatment in MT. Interval from PGF to estrus was similar between treatments in MT (65.5 ± 1.6) but was greater (P < 0.05) in the YF (78.5 ± 1.4) than MF (53.6 ± 2.2) treatment in OH (trt x loc, P < 0.01). Estrous response (89%) and conception rate did not differ (P > 0.10) for MF (64.1%) and YF (67.2%) heifers. The MF2 heifers in MT had a greater (P < 0.01) interval to estrus and smaller (P < 0.01) ovulatory follicles at AI than MF heifers with a single follicular wave after EB. When MF2 heifers were removed from analyses, interval from PGF to estrus in MT was greater (P < 0.01) but follicle diameter was smaller (P = 0.01) in YF than MF heifers. Furthermore, this interval was similar for MF heifers between locations (54.6 ± 1.7), but was greater (P < 0.01) for YF heifers in OH (78.5 ± 1.4) than MT (67.4 ± 1.5). In conclusion, manipulation of age of the ovulatory follicle at spontaneous ovulation did not influence conception rate.